The anatomy of the knee is primarily composed of two major bones, the tibia and the femur (see FIG. 1). These two bones meet to form a joint between the proximal tibia and the distal femur. A third member of the joint is the patella, or the kneecap. The knee joint is cushioned by articular cartilage that covers the ends of the tibia and femur, as well as the underside of the patella. The lateral meniscus and medial meniscus are pads of cartilage that further cushion the joint, acting as shock absorbers between the bones.
A healthy knee joint is able to withstand great forces that are exerted as the knee flexes and extends and supports the weight of the body. However, when the knee joint degenerates and becomes arthritic due to an injury or infection of the joint or due to aging, it may become necessary to reconstruct or replace the knee joint. When a total knee replacement (arthroplasty) is required, the natural knee joint is replaced with a prosthetic knee joint. A certain percentage of patients who undergo a total knee replacement surgery eventually contract an infection in the knee joint at the surgical site necessitating further surgery for control of the infection.
Two stage reimplantation total knee arthroplasty has become an increasingly common operation for the treatment of infected total knee arthroplasty. The treatment of infected total knee arthroplasty requires the removal of the old prosthesis (artificial joint) and placement of a spacing device to treat the infection before a new prosthesis is implanted. The spacing device is needed, since it is important to maintain the original space between the joints in order to prevent a reduction in the articular space (see FIG. 2), retraction of the tissues, atrophy of the joints and loss of muscular elasticity as well as tone. In addition, the spacing device contains antibiotics that elute from the spacer after implantation, locally delivering antibiotics to the site of the infection. This is an essential part of the overall treatment to eradicate the infection.
Non-articulating as well as articulating spacing devices are known in the art for artificial knee, hip and shoulder joints. Traditional non-articulating, fixed spacing devices for the knee are applied as a bone cement block that is mixed with antibiotics by the surgeon and is placed between the tibia and the femur. Though these fixed spacing devices have been shown to be effective in treating infections and maintaining space for reimplantation of a second knee replacement after the infection has been eradicated, these spacers do not allow for knee movement. As treatment for infection after knee replacement can take many months, it is very inconvenient for the patient to not be able to move his knee for such an extended period of time.
Prefabricated and preformed articulating spacing devices made of bone cement and other materials exist that allow the patient limited mobility (e.g. Interspace® knee spacer from Exactech, Gainesville, Fla.), but, as with limited size options, do not provide an optimal fit to the particular bony anatomy of each patient. In addition, preformed articulating spacing devices do not give the surgeon the flexibility to select the type and concentration of antibiotic in the spacer. Since different bacteria have different sensitivities to different antibiotics, this is a significant disadvantage.
Other existing technologies (e.g. Stage One molds, Biomet Inc.) involve the surgeon using two preformed molds to make articulating spacers. With these technologies, the surgeon can select the type and concentration of antibiotic in the cement, but the molding process occurs outside of the patient in two separate molds, one mold to create the femoral spacer component, and the other to create the tibial spacer component. The surgeon must then wait for the cement to harden, remove the spacer from each mold, and then either place these two molded articulating spacer components in the patient, and not achieve a custom fit with the irregular bone surface of the patient, or the surgeon must then mix a second batch of cement to cement these spacers on the patients bone. Though this solution provides the surgeon with the capability of selecting the type and concentration of antibiotic in the spacer, the only way to achieve a custom fit with the irregular bone surface of each patient is to mix two batches of cement, one to make the molded components and the other to apply them to the existing irregular bone surfaces of the patient, a process that takes considerable valuable operating room time.
Another proposed solution to this problem involves placing sterile metal and plastic knee replacement components in the knee with cement that has been mixed with antibiotics. This solution is controversial as it involves placing a large quantity of inert material (metal and plastic) that does not elute antibiotics into the infected site.
Reproducing the knee joint using custom-fit temporary spacing devices that have only antibiotic eluting material and that simulate the natural shape of the tibial and femoral components of the knee joint would be superior to the fixed spacing devices, since they provide an optimal fit to the particular bony anatomy of a patient, permit movement of the leg, and can still deliver antibiotics to aid in the treatment of infection and maintaining space for future surgery. There is a clear need in the art for custom-fit articulating spacers that provide the surgeon with the ability to deliver a specific antibiotic at a specific concentration, that only require one round of mixing cement, and that do not require post-molding modification.
The present invention fully addresses this need.